Xerographic printers, such as plain paper office copy machines, produce images by use of a finely divided, colored dielectric toner powder which is deposited in the form of an image in conformity with an electrostatic latent image carried by an imaging surface. Once formed, the powder image is transferred to a final support sheet and residual toner is cleaned from the imaging surface to prepare it for reuse. The cleaning operation is accomplished by mechanically dislodging the toner particles from the imaging surface and entraining the toner particles in an air stream.
Many cleaning systems in use resemble a household vacuum cleaner employing a brush that dislodges the particles and lifts them into a moving air stream where they are wisked away from the surface being cleaned. The cleaning process may be enhanced by a pre-clean charging treatment and light exposure that provide optimum electrostatic conditions for the cleaning operation. Like a household vacuum cleaner, the toner laden air is ordinarily passed through a cloth-like filter where the toner particles are mechanically trapped to clean the air prior to discharge from the machine. When loaded with toner, the filter bag is usually replaced and discarded.
Electrostatic precipitators have been proposed for cleaning the toner from the air. Such devices provide a strong electrostatic field that tends to drive the toner to a moving surface from which it can be scrapped and collected for re-use. Electrostatic precipitators tend to add cost by requiring both mechanical and electric additions to the copy machine system.
I have discovered that the same carrier beads that interact triboelectrically with the toner material in the developing station to charge the toner material will, if collected in a body through which toner laden air can pass, thoroughly filter the toner particles from the air. The unusual effectiveness of this carrier as a filter material can be explained by the relatively large holding force that is developed upon contact between a toner particle and the carrier bead surface due to the triboelectric relationship between the materials of the particle and the surface. This holding force is substantially greater than the forces placed on the toner particle to cause it to pass through the interstices between adjacent carrier beads. Thus the force of the air stream that brings the toner particle into contact with a carrier bead is substantially less than that necessary to dislodge the toner particle from a carrier bead which it has contacted. This holding force is also large in comparison with forces that might be developed due to the initial charge on the toner and a carrier bead prior to actual contact therebetween, since much of the field that would be created by initial charge within a body of carrier beads tends to be negated by similar charge from adjacent carrier beads.
The ability of developer carrier beads to act as a filter material greatly simplifies the reclamation of toner particles since the toner laden carrier of a saturated filter body can simply be mixed into the developer mix from which the toner originally came.